The present invention relates to computer controlled robot arms and, more particularly, control systems for positioning and holding work heads carried by robot arms at selected coordinate positions for performing desired operations within close tolerance limits.
Computer controlled robot arm assemblies are becoming more widely used in a variety of assembly line operations. In uses such as the drilling or spot welding of automobiles, precision is not important. Thus, the tolerance limits capable because of the play in the various joints of the robot arm are not critical.
As reported in the Mar. 1, 1982 edition of Aviation Week and Space Technology in an article beginning at page 69 entitled "USAF Seeks Increased Robot Precision," present robot arm assemblies are not capable of maintaining the accuracy required in the fabrication of aircraft parts. For example, as stated therein, an overlay template with tapered lock-on bushings is required to achieve needed positioning accuracy for drilling of the acrylic windshields on the McDonnell-Douglas F-15 Aircraft. The fabrication of such templates prior to production runs is, of course, time consuming and expensive. Any changes to the specification requires a remake or modification of the template. Another problem is the type of materials being used in the more exotic high-speed aircraft. As reported in the aforementioned article, lack of rigidity at the end of a robot's arm can cause problems in the shape of holes drilled by robots in certain types of material such as graphite epoxy composites. If a robot's hand effector or work head fails to hold a drill steadily perpendicular to the materials surface, the hole may become cloverleafed or egg-shaped. The specifications for holes required in aerospace manufacturing include class 2, which must be no smaller than the nominal size and a circumference with bulges no larger than 0.003 inches and class 3 holes which must be within 0.006 inches. What is desired is a robot arm assembly with tool-placement accuracy of 0.005 inches.
The problem can be visualized in the simplified drawing of a prior art robot arm assembly shown in FIG. 1. The assembly, generally indicated as 10, comprises a base 12 having a number of powered, articulated arms 14 extending therefrom and terminating in an effector or work head 16 at the free end thereof. A computer 18 is connected to move the arms 14 to position the work head 16 at various locations. For example, as shown, a work piece 20 can be positioned on a work table 22 and a drill bit 24 driven by the work head 16 moved about the work piece 20 to drill the required holes therein at preselected locations. As can be seen, however, a certain amount of vertical play as indicated by the angle "A" occurs at the first joint labelled 26. A certain amount of horizontal play as indicated by the angle "B" also occurs at the first joint 26. At the second joint, indicated as 28, there is additional vertical play as indicated by the angle "C" and horizontal play as indicated by the angle "D". At the work head 16, the play in the joints 26, 28 translates into tolerance errors in the X, Y and Z directions. If the coordinate directions are taken as indicated in FIG. 1, it can be seen that the horizontal combination of play of angles B and 2 translates into a X coordinate tolerance limit of L(B+D). In like manner, the Y tolerance limit is M(A+C) and the Z direction (which in this case effects the depth of the hole drilled) has a tolerance of K(A+C).
While the play movements at the joints 26, 28 is, admittedly, small, the accumulation of tolerances is such as to prevent the robot arm assembly 10 according to the prior art from achieving the necessary tolerance limits discussed above, particularly when the robot arms are relatively long or flexible. Wherefore, it is the object of the present invention to provide a robot arm assembly capable of independently achieving high tolerance in position achievement and maintenance without the necessity for overlay templates and the like.